Two recent technological advances have made large, flat pnel display devices a distinct practical possibility: (1) the preparation of liquid crystals exhibiting improved physical properties; and (2) the production of surface layers of fine-grained polysilicon.
The development of liquid crystals has progressed sufficiently such that miniature color television receiver sets can be made from them and much larger information display panels are technically feasible. However, liquid crystals inherently demonstrate slow responses to electrical signals and, therefore, a "switch" to rapidly respond to an electrical stimulus is required to set up an active matrix display. The thin film transistor (TFT) performs that function.
It is well recognized that a TFT can be fabricated from single crystal silicon; nevertheless, by the very nature of single crystal silicon, there is a limitation on the size of a TFT that can be prepared therefrom. It has been discovered that a fine-graind polysilicon layer which has been deposited onto a substarate can be recrystallized to large-grained polysilicon through the scanning thereof with a heat source such as a laser. Further, it has been observed that a TFT produced from large-grained polysilicon manifests only slight deterioration in electrical properties when compared with those same characteristics exhibited by a TFT fabricated from single crystal silicon, and performs very satisfactorily in multiplexing a liquid crystal display.
A larger-grain polysilicon active matrix display requires a substrate which is transparent, flat, smooth, inert, compatible with silicon with respect to thermal expansion, and is capable of withstanding processing temperatures of at least 850.degree. C. and, preferably, much higher, i.e., up to a temperature approaching 1000.degree. C. U.S. application Ser. No. 790,369, filed concurrently herewith by me under the title STRONTIUM ALUMINOSILICATE GLASS SUBSTRATES FOR FLAT PANEL DISPLAY DEVICES, discloses glasses within a very narrow range of the ternary system SrO--Al.sub.2 O.sub.3 --SiO.sub.2, viz., glasses consisting essentially, expressed in terms of mole percent on the oxide basis, of 9-12% SrO, 9-12% Al.sub.2 O.sub.3, and 77-81% SiO.sub.2, which, having annealing points in excess of 850.degree. C., and, preferably, in excess of 875.degree. C., are useful as substrates for large-grained polysilicon active matrices. Such glasses have, indeed, been recommended as operable substrates. However, as the technology progresses, it has been appreciated that resistance to even higher processing temperatures may quite likely be demanded. Consequently, glass forming compositions exhibiting linear coefficients of thermal expansion over the range of 25.degree.-300.degree. C. between about 30-40.times.10.sup.-7 /.degree.C. and being capable of withstanding processing temperatures in excess of 900.degree. C. and up to 1000.degree. C. would be highly desirable.
In view of the above, the primary objective of the present invention is to provide a transparent glassy material chemically inert to silicon, but which is compatible with silicon with respect to thermal expansion through a linear coefficient of thermal expansion over the range of 25.degree.-300.degree. C. between about 30-40.times.10.sup.-7 /.degree.C., and which exhibits an annealing point higher than 900.degree. C. and, preferably, higher than 950.degree. C.